Field of the Invention
The present invention relates to a cladding material for a battery collector and an electrode employing the cladding material for a battery collector.
Description of the Background Art
A cladding material for a battery collector manufactured by bonding layers constituted of an Al-based alloy and a Cu-based alloy respectively to each other is known in general, as disclosed in Japanese Patent Laying-Open No. 8-7926 (1996), for example.
Japanese Patent Laying-Open No. 8-7926 discloses a bipolar lithium-ion secondary battery including a cladding material (a bipolar electrode) for a battery collector consisting of a cladding material manufactured by rolling aluminum foil having a thickness of 20 μm and copper foil having a thickness of 10 μm. The ratio of the thickness of the aluminum foil to the total thickness of the aluminum foil and the copper foil is about 67% (=20/(20+10))×100).
In the cladding material for a battery collector disclosed in Japanese Patent Laying-Open No. 8-7926, however, the side of the aluminum foil is elongated beyond the side of the copper foil when the aluminum foil and the copper foil are bonded to each other by rolling, since the aluminum foil more easily plastically deformable than the copper foil has the large thickness ratio of about 67%. Consequently, the cladding material for a battery collector is disadvantageously remarkably warped to be convexed on the side of the aluminum foil. When the cladding material for a battery collector is heat-treated, annealed and thereafter cooled to the room temperature, thermal shrinkage of the aluminum foil having a large thickness remarkably influences at the time of the cooling after the heat treatment, since the thermal expansion coefficient of the aluminum foil is larger than that of the copper foil. Therefore, the cladding material for a battery collector is also disadvantageously remarkably warped to be concaved on the side of the aluminum foil.
In order to solve the aforementioned problems, Japanese Patent Laying-Open No. 2010-33768 proposes a bipolar lithium-ion secondary battery including a collector consisting of stainless foil having a roughened surface with a thickness of not more than 50 μm. The collector consisting of the stainless foil proposed in Japanese Patent Laying-Open No. 2010-33768 is not a cladding material, and hence the same can be prevented from remarkable warps at the time of rolling or cooling after heat treatment.
While the collector consisting of the stainless foil described in Japanese Patent Laying-Open No. 2010-33768 can be prevented from remarkable warps, however, the stainless is a hard metallic material. When a slurry containing an active material is applied to the roughened surface of the collector, dried and thereafter solidified so that the active material is in close contact with the roughened surface through a step of applying pressure by roll pressing, therefore, the active material does not sufficiently penetrate into the surface of the collector. Therefore, the active material cannot be sufficiently brought into close contact with the collector, and hence it is conceivably difficult to increase the contact area between the collector and the active material. Consequently, contact resistance between the collector and the active material is conceivably increased. In general, therefore, it is difficult to prevent the collector from remarkable warps and to suppress increase in contact resistance between the collector and the active material.